1. Field of the Invention
The field of the invention is electronic, motor drives for variable speed control of AC induction motors, and more particularly, motor drives which do not utilize direct speed sensing through a tachometer, position encoder or resolver.
2. Description of the Background Art
In high performance drives with speed sensing, speed feedback from the rotor is used in controlling the torque and speed at which the motor is operated. Motor parameters such as inductance (L) and resistance (R) of stator and rotor windings have been used as inputs for tuning the current and speed-torque control loops.
While techniques are known to reduce the number of motor parameters which must be evaluated, they have not altogether eliminated some motor parameter inputs in a high performance drive. The motor parameters must be obtained from the manufacturer of the motor or determined through rigorous testing of the motor.
Lower cost drives have included controls of the open loop type in which external speed sensing equipment is not utilized and in which knowledge of motor parameters such as inductance (L) and resistance (R) is not required. Such a drive has been disclosed in Rowan et al., Ser. No. 07/326,483 filed Mar. 20, 1989, and entitled "Open Loop Motor Control with Both Voltage and Current Regulation." This drive Provided an inner current regulator loop to control stator current. An outer voltage control loop was used to control stator voltage by controlling the command fed to the current regulator loop. The voltage controller generated a voltage command based on user-specified volts/hertz and accel/decel parameters. This command was compared with the actual motor voltage which was sensed at the outputs of the current regulator. The difference between the commanded and actual voltage became an error signal for controlling the current command signal.
When a motor is subjected to a sudden and substantial increase in load or when the motor drive is reconnected to a motor under load, the drive may encounter transients that cause nuisance tripping (interruption of power). These conditions are encountered in motor control applications in the wood cutting, metal forming and fiber and paper industries.
High performance motor controls provide some type of torque, speed or slip control to handle the special conditions described above. It is therefore an object of the present invention to provide an open loop control with improved response to these conditions.